In many clinical settings, aspiration is probably the most common cause of pulmonary pathology. The actual incidence of aspiration is speculative, because often it occurs unobserved and the clinical response is non specific. Two clinical tests have been developed to detect aspiration. One involves placing Evans Blue Dye intraorally and looking for signs of it in tracheal secretions obtained in patients with tracheostomies. The second utilizes a barium suspension of tantalum placed intraorally, with subsequent oblique x-rays of the trachea to detect aspiration. Utilizing these two tests the incidence of aspiration, and means of decreasing it, will be documented in various clinical settings and diseases. The treatment of massive aspiration pneumonia is largely symptomatic, and carries a high mortality. The pathogenesis has been poorly understood, and therefore treatment has not been based on a firm foundation. Using the isolated, perfused, ventilated canine lobe the pathophysiology of the acid injury to pulmonary tissue will be followed. Subsequently, the effects of various forms of treatment on the response of the isolated lobe following acid-instillation will be evaluated. In addition, studies will be carried out studying the effects of small amounts of aspiration on mucociliary clearance rates in dogs.